This invention relates generally to telecommunications and more particularly a linear transmitter that combines feedback linearization with envelope modulated signal formats.
The world-wide use of transmitters employing linear modulation with a non-constant envelope has created a need for improved power efficiency in transmitters. Because of this need, supply modulation is applied to the final stages of transmitters.
In existing linear transmitters employing feedback linearization in combination with supply modulation, the incidental phase modulation occurring in the final stages of the transmitter induced by the supply modulation can reduce the normally adequate phase margin level in the system dramatically. Also, in existing systems, supply modulation is combined with various linearization topologies. The envelope component may selectively be mapped to provide optimum power efficiency versus drive level and to have a reduced bandwidth prior to its application as supply modulation.
Disadvantageously, applying supply modulation in the final stages of a feedback-linearized transmitter system creates an incidental phase modulation that can create highly-peaked closed-loop frequency responses that put excessive noise into neighboring channels. Also, applying supply modulation in the final stages of feedback-linearized transmitters can create outright instability.